Boats are generally moored to a dock using a mooring device such as a mooring line and cleat. While moored, a boat still moves with changes to the level of the surface of the water. This motion is especially pronounced when a boat is hit by a sizable wave or when a boat is moored for a long enough period that it feels the effects of the changing tide.
In order to compensate for these changes in the water's surface, boat owners leave some slack in their mooring lines. This slack, however, allows the boat to pitch and yaw which can result in the boat colliding with the dock, nearby boats, or other nearby items. This can cause damage to the boat, the dock, or nearby items. To diminish the likelihood of such damage, some boats and docks use foam or rubber bumpers. These bumpers, however, are generally stationary, meaning that they do not adjust to the height of the surface of the water relative to the boat. Such stationary bumpers cannot prevent damage associated with significant changes in water level.